The objective of this research is an in situ study of the cholinergic and adrenergic receptor systems in human erythrocytes. Erythrocyte response to neurotransmitters resembles smooth muscle contraction in many respects. Investigation of excitation and contraction mechanisms in these simple, discrete, easily manipulated cells may provide findings relevant to function and pathology in experimentally inaccessible cells such as those in vascular and respiratory smooth muscle. The composition, physical disposition, and chemistry of these neurotransmitter receptor systems are being studied using drug binding, crosslinking, fluorescence and electron spin resonance spectroscopy, selective extraction and partial reconstitution techniques.